Numerous proposals exist for producing gases containing carbon monoxide, hydrogen and, optionally, carbon dioxide. For purposes herein, these gases are referred to as synthesis gas (syngas). Syngas can be produced from virtually any carbon-containing feedstock including, but not limited to, biomass and fossil fuels such as natural gas, petroleum and coal. Processes for generating syngas include, but are not limited to, gasification, partial oxidation and reforming (autothermal and steam). Other sources of the gas substrate include gases generated during petroleum and petrochemical processing and off-gases from manufacturing operations including, but not limited to, blast furnace and coke oven operations, the steel industry, non-ferrous metal industries, or captured gas from incomplete combustion processes.
Syngas can be a fuel but is often used as an intermediate for the production of other chemicals such as ammonia, methanol, and synthetic petroleum via the Fischer-Tropsch process. Syngas can also be bioconverted to produce alkanols, diols, carboxylic acids and esters, and alkanes such as methane.
Impurities in syngas generally exist due to the source of the syngas and the raw materials used to generate the syngas. Examples of impurities that can exist in syngas include, but are not limited to, acetylene, ethylene, hydrogen sulfide, carbonyl sulfide, carbon disulfide, nitric oxide, and hydrogen cyanide. Some of these impurities can be deleterious to the intended application of the syngas and accordingly must be removed. In bioprocesses, certain impurities can be inhibitors to one or more metabolic pathways or can be lethal to the microorganism. See, for instance, Xu, et al., The Effects of Syngas Impurities on Syngas Fermentation to Liquid Fuels, Biomass and Bioenergy, 35 (2011), 2690-2696; United States Published Patent Application No. 20110097701; Abubackar, et al., Biological Conversion of Carbon Monoxide: Rich Syngas or Waste Gases to Bioethanol, Biofuels, Bioproducts & Biorefining, 5, (2011), 93-114; and Munasinghe, et al., Biomass-derived Syngas Fermentation into Biofuels: Opportunities and Challenges, Bioresource Technology, 101, (2011), 5013-5022. Particularly deleterious impurities to microorganisms are hydrogen cyanide and acetylene. Hydrogen cyanide is also deleterious to catalysts used for Fischer-Tropsch processes.
U.S. Pat. No. 4,189,307 discloses a process for the removal of hydrogen cyanide from syngas by absorption in water. European Patent 1 051 351 B1 discloses processes for the removal of ammonia and hydrogen cyanide from syngas. In the disclosed process, hydrogen cyanide is converted to ammonia and is removed from the gas with water. A hydrocarbon gas used as the feedstock to the synthesis gas generator is used to strip ammonia out of the water. A catalytic hydrolysis or hydrogenation is used to convert the hydrogen cyanide to ammonia. It is stated at page 3 that “ . . . the concentration of the combined total hydrogen and ammonia present in the syngas is preferably reduced to less than 0.1 vppm . . . ” The disclosed process is complex and requires a catalyst which may be deactivated. Indeed, the patent discloses at column 5, lines 4 et seq., the use of substantially sulfur free methane.
U.S. Pat. No. 8,303,849 discloses processes for removing hydrogen cyanide from syngas using chemical and biological treatment. The process uses an aqueous scrubbing unit operation under certain conditions and then subjects the aqueous solution to microbial activity to degrade the hydrogen cyanide.
Processes are still sought to remove hydrogen cyanide from syngas, especially to concentrations of less than 1 part per million by volume in an economically attractive manner. More desirably, such processes would degrade hydrogen cyanide. Even further advantageous processes would remove other contaminants from the syngas.